Along with recent demands for downsizing and thickness reduction of an electronic device, requirements increase for a smaller feature size of a semiconductor element to be mounted on the electronic device. Conventionally, in lithography, to manufacture a semiconductor element, reduction projection exposure using ultraviolet rays is performed. The minimum size that can be transferred by reduction projection exposure is proportional to the wavelength of light used for the transfer, and inversely proportional to the numerical aperture of a projection optical system. Hence, in order to transfer a smaller microcircuit pattern, the wavelength of exposure light to be employed decreases, such as a mercury lamp i-line (wavelength: 365 nm), a KrF excimer laser (wavelength: 248 nm), and an ArF excimer laser (wavelength: 193 nm).
The feature size of the semiconductor device is decreasing rapidly, and lithography using ultraviolet light has limitations in dealing with the small feature size. In order to efficiently transfer a very small microcircuit pattern of as small as less than 0.1 μm, a projection exposure apparatus which uses extreme ultraviolet light (EUV light) having a wavelength of about 10 nm to 15 nm, which is much shorter than that of ultraviolet rays, has been developed.
As an EUV light source, for example, a laser plasma light source is used. The laser plasma light source irradiates a target member placed in a vacuum container with a high intensity pulse laser beam using a YAG laser, or the like, to generate a high temperature plasma, so as to utilize EUV light having a wavelength of, e.g., about 13.5 nm, which is generated by the YAG target. As the target member, a thin metal film, inert gas, droplets, or the like, is used, and is supplied into the vacuum chamber by a means such as a gas jet. To increase the average intensity of the EUV light radiated from the target, the higher the repetition frequency of the pulse laser, the better. Usually, the EUV light source is operated at a repetition frequency of several kHz.
When the target is irradiated with the high-intensity pulse laser beam, the target generates not only EUV light, but also scattering particles called debris. If the debris attaches to an optical element, it may contaminate, damage, and decrease the reflectance of the optical element. Hence, a debris removing means which prevents the debris on the target from reaching the optical element has been employed.
In order to facilitate the debris to flow into an illumination optical system, preferably, an EUV light condenser mirror is an elliptical mirror, which has one focal point at a point where a plasma is generated and focuses light on the other focal point. Preferably, the path that connects a light source portion and an illuminating portion is physically narrow.
In an exposure apparatus shown in FIG. 6, each of the substrates of a condenser mirror 508, mirrors that constitute an illumination optical system 520, a mask 521, and mirrors that constitute a projection optical system 522 have several tens of pairs of multilayered films made of Mo, Si, and the like, in order to reflect EUV light 506 efficiently. Each multilayered film must have a surface roughness on the Å order by a standard deviation in order to suppress a decrease in reflectance of the EUV light. Even so, the actually obtained reflectance is about 60% to 70% at most. Namely, the remaining 30% to 40% of the EUV light is absorbed by the mirror and converted into heat to undesirably increase the temperature of the substrate. As an example, assume that the respective mirrors uniformly have reflectances of 65% and that the EUV light is reflected thirteen times, including reflection by the illumination optical system 520, mask 521, and projection optical system 522, 0.6513=0.0037 of the light reaches the surface of a wafer 523. In other words, only 0.37% of the EUV light extracted from the light source portion reaches the surface of the wafer 523, and the remaining 99.63% of the light is absorbed by the constituent elements, i.e., the illumination optical system 520, mask 521, and projection optical system 522. As is apparent from this example, in an EUV exposure apparatus, an optical system that satisfies a desired optical performance with a smaller number of mirrors must be achieved.
In order to achieve the desired optical performance, for example, in order to suppress the decrease in reflectance, the mirrors of the projection optical system 522 must have, not only a smaller surface roughness, but also shape accuracy on the Å order by standard deviation. A very accurate optical system is thus required. Accordingly, the stability of the optical system 522 against external factors, such as the temperature, must naturally be very high. As a result, sufficient consideration must be made against heat obtained by conversion of absorbed EUV light, as described above.
In order to improve the productivity of the exposure apparatus, as much EUV light as possible must be supplied onto the surface of the wafer 523. To achieve this, the reflectances of the respective mirrors must be increased, the number of mirrors to be used must be decreased, and simultaneously, the EUV light to be output from the light source portion must be increased. In this case, the mirrors of the illumination optical system 520, which are close to the light source portion, receive a large quantity of EUV light and absorb 30% to 40% of the EUV light in the form of heat. While the illumination optical system 520 does not require a shape accuracy as high as that required by the projection optical system 522, it is exposed to the intense EUV light output from the light source portion. Thus, sufficient consideration must be made against the quantity of heat absorbed by the illumination optical system 520, which is greatly larger than that of heat absorbed by the projection optical system 522.
As the EUV light is readily absorbed by the atmosphere, the optical path space for it must be a vacuum environment, as described above. Therefore, to remove the heat absorbed by the mirrors, a cooling system employing heat exchange with a temperature-regulated gas, which is generally used in a conventional exposure apparatus cannot be employed. In the case of an EUV exposure apparatus employing a vacuum atmosphere, a temperature regulating medium must be directly brought into contact with the mirrors to remove by heat transfer the heat absorbed by the mirrors. Alternatively, heat absorbed by the mirrors must be removed by radiant heat using a vacuum. In any case, because of the vacuum environment, the optical system cannot be entirely temperature controlled by temperature regulated gas. Hence, a temperature regulating systems must be provided for the respective mirrors or each for several mirrors individually.
In the EUV exposure apparatus, the system having the individual temperature regulating systems for the respective mirrors or each of the several mirrors is a basic constituent element considering heat absorption by the mirrors, the high accuracy demand for the mirror shapes, and the vacuum environment on the optical path.
EUV exposure apparatuses are described in Japanese Patent Laid Open No. 2004-193468 and No. 2001-143992.
The optical path is set in a vacuum environment in order to exclude an atmosphere, which absorbs the EUV light. The EUV light has a very short wavelength, i.e., 13.5 nm in the soft X-ray range. Even in the vacuum, the mirror surface is contaminated by a contaminant due to the mutual operation of the remaining components on the mirror and EUV light to not only degrade the reflectance, but also degrade the uniformity. Consequently, good exposure is interfered with.
Contamination is roughly divided into two types, i.e., oxidation of a multilayered mirror film caused by the presence of water and contamination due to carbon, and is caused when the remaining components of water or carbon exist. The allowable amount of the residual components is said to be about 1×10−7 Pa by partial pressure for water and about 1×10−8 Pa by partial pressure for carbon molecules having a molecular weight of forty-four or more. A very strict vacuum degree is thus required. For this reason, that member to be formed in the vacuum chamber of the exposure apparatus, which causes outgassing must have a surface area as small as possible, and must be made of such a material or be subjected to such a special process that outgassing of a gas containing a material, which generates the partial pressure, does not occur. To decrease the surface area which causes outgassing and to decrease the special material or process is very significant in apparatus design, because they are directly related to the apparatus cost.
Regarding cleaning and removal of a contaminant once attaching to the mirror, intensive studies are made on cleaning and removing carbon while maintaining the surface roughness on the Å level. Once the contaminant is oxidized, however, it cannot be cleaned or removed, and no means is available, but to exchange the mirror itself when the reflectance of the mirror decreases by a certain degree.
Regarding the decrease in reflectance, the rate of decrease of the entire optical system ranging from the illumination optical system 520 to the projection optical system 522 can be detected by mounting an exposure dose sensor (not shown) on the wafer stage and measuring the reflectance periodically. This is the same as in a conventional KrF exposure apparatus having a wavelength of 248 nm and a conventional ArF exposure apparatus having a wavelength of 193 nm.
With this sensor, however, which mirror has the problem cannot be identified, and an appropriate measure cannot be taken against the inconvenience of the decrease in reflectance, so the apparatus downtime increases. Particularly, as the EUV exposure apparatus employs a vacuum environment, if the apparatus is open to the atmosphere in order to inspect a component, or the like, in the apparatus, restoration of the apparatus takes a considerable amount of time, on the order of many days, thus adversely affecting the productivity of the apparatus.
In order to avoid this problem, a new detection system which detects a decrease in reflectance of the mirror or the contamination itself may be formed in the optical path. In this case, to achieve a clean, high vacuum degree with few residual components that cause contamination, as described above, such a new detection system, which may lead to an increase in outgassing should not be formed in the vacuum chamber, if it is possible.